White gold and palladium

I have a customer at the moment wanting an 18K white gold band with
an overlay of palladium. She’s looking for a slightly grey colour
difference in the palladium so the grey will contrast the whiter
colour of the shank.

The 18K I normally use has palladium as part of the alloy. Is there
a pall alloy that will give her this colour difference? Do I need to
mix it myself and is this an easy thing to do?

Thanks. Colin.